I've got some 10 years old film transfers that were done in SD. I want to position the image in the middle of at 16:9 1080p HD frame. The destination is Vimeo and the NLE is Premiere Pro. Thanks in advance for your collective expertise.

I do this all the time in Adobe Premiere. Simply create a new project and a new sequence with your 720x480 (DV NTSC?) as resolution. Drag the videofile onto the timeline. Upon export, choose H.264 and choose one of the HD presets. There might even be one for Vimeo. Your frames are centered and you will have black pillar boxes on each side. You should see a preview with the image in the center and black boxes on each side.